This invention relates to a system and method for catalytically oxidizing volatile organic compounds (VOC""s) utilizing the exhaust of an internal combustion engine.
Controlling air pollution is acknowledged as a nationwide and worldwide concern. A major contributor of air pollution is attributed to the release of volatile organic compounds into the atmosphere. Accordingly, the reduction of VOC emissions is considered an important part of overall pollution control management to improve air quality.
Common VOC""s such as pentane, ethanol, methanol, ethyl acetate, hexane, as well as others are produced from manufacturing sites by solvent vaporization or as by-products. Accordingly, in most cases, the production of VOC""s are being unavoidably formed and need to be treated or destroyed to satisfy the environmental requirements of most of the developed nations in the world.
Thermal incineration and catalytic oxidation are the most frequently used control methods for destroying VOC""s. In the combustion method, waste gas containing VOCs is mixed with natural gas and combusted in a large chamber into which sufficient natural gas is fed to sustain combustion with carbon dioxide and water vapor formed as the primary products of the combustion. Incineration of VOC""s requires significant amounts of electricity and natural gas and although some energy can be recovered as heat the operating cost of combustion type devices is high and, as such, is not a cost effective method for treating VOC""s.
One method which has been considered in the prior art for offsetting the high operating cost of combustion is to perform the combustion in an internal combustion engine in which the heat of combustion is used to assist in the generation of electricity. This is taught in U.S. Pat. No""s.: 5,592,811; 5,718,112 and 5,592,813 respectively. These patents use the conventional combustion principal to thermally incinerate the VOC""s in the internal combustion engine while utilizing the output heat of combustion to improve the efficiency of the internal combustion engine for generating electricity. Because of the need for a relatively long residence time to combust VOC gases in an internal combustion engine this process is inherently inefficient without substantial modification of the engine to insure adequate residence time for combustion to occur so as to fully burn the VOC""s. The process is also dependent on very high VOC concentrations in the effluent VOC stream. For small plants with VOC emissions substantially below 1000 ppm and particularly for VOC concentration levels below 100 ppm the use of combustion type devices for thermal incineration of the VOC""s does not increase the operating efficiency of the combustion device and does not decrease the overall cost of operation.
Catalytic oxidation uses a catalytic bed of metal catalysts such as platinum, palladium, platinum/rhenium, etc., to oxidize the VOCs. Since the catalytic oxidation process requires an operating temperature of around 450 to 750xc2x0 F. for destruction efficiency of greater than 95% as required by the clean air regulation, it is conventional to use a gas burner to pre-heat the VOC-containing waste gas to the required operating temperature before passage through the catalytic bed. The catalyst inlet temperature required for 95% destruction is dependent on the type of VOC in the stream. A gas burner is expensive to purchase, operate and maintain. The high operating cost is attributable primarily to the supply of natural gas and to the cost of maintenance. In addition, burners cannot be used in many pharmaceutical and chemical plants due to flammable solvents used in such plants. While turbines or reciprocating engines are usually permitted in such plants, gas burners used in the conventional catalytic oxidation unit are not allowed.
A common approach to these problems is installing a single, large-scale waste gas treatment unit in a remote place for an entire facility. Waste gas produced from each individual workstation is transferred through pipelines and consolidated to one gas steam, which is, then, introduced to the waste gas treatment unit. Such centralized waste gas treatment unit operates using a gas burner, a burner safety controller, and a large-scale catalytic oxidation unit for the entire collected waste gas. Although bigger and more expensive burners and larger-scale catalytic oxidation units are used for such centralized systems, overall cost thereof is much less than those for individual installations at several work units. The economic benefit of the centralized waste treatment system is mainly derived at the expense of operation of individual workstations. However, such centralized system require continuous operation even when only few workstations are in operation and for maintenance or repair, the entire factory has to be taken off-line. Consequently, the economics of catalytic oxidation is primarily attributable to the operational cost of the gas burner.
It has been discovered in accordance with the present invention that catalytic oxidation can be used for the destruction of volatile organic compounds in a waste effluent even at very low VOC concentration levels without the need for a gas burner. This is accomplished by utilizing the exhaust from a conventional internal combustion engine as a heat source to achieve the optimum temperature for the catalytic oxidation of the VOC""s while simultaneously employing the electrical power generated from the internal combustion engine to offset the cost of fuel for operating the internal combustion engine. Moreover, since an appropriate optimum temperature for the inlet gas for the catalytic oxidation can be maintained by re-cycling the heat from the catalytic oxidation to a heat exchanger through which at least a substantial portion of the waste VOC effluent is passed the overall thermal energy required from the exhaust of the internal combustion engine can be significantly reduced. Consequently, substantial reduction of operating cost of such waste gas treatment units can be achieved. Furthermore, since this invention does not require a gas burner overall installation and operating costs are substantially reduced. Generally, the burner used in the conventional method requires specialized maintenance in order for effective and prompt operation, which influence the uptime of the catalytic oxidation process. This invention, however, can ensure the uptime of the oxidation process without special care because operation of a power generator are usually guaranteed by the manufacture with greater than 95% uptime. This invention also reduces costs related to VOC flow control devices such as stream direction control values and pipes since the catalytic oxidation units need to be in relative close proximity to the source generating VOC.
The method of the present invention for destroying volatile organic compounds (VOCs) broadly comprises:
passing all or a significant portion of an effluent VOC containing gas stream through a heat exchanger for preheating the VOC containing gas stream;
mixing said preheated VOC-containing gas stream with the exhaust gas from an internal combustion engine operating from a fuel supply to raise the temperature of the mixed gas to a temperature above at least about 500xc2x0 F.;
passing said mixed gas through a bed of catalysts for catalytically oxidizing said VOC""s; and
directing the output from said bed of catalysts to said heat exchanger.